Debug worlds generate one column twice

To reproduce, create a debug world and look at the blocks from coordinates 3 70 1 to coordinates 175 70 1 and compare them with those at the other side (1 70 179 to 173 70 179).

In fact, you can do this using /testforblocks: First, teleport to the middle so the whole grid is loaded:

/tp @p 83 113 71

Then, compare the blocks:

/testforblocks 3 70 1 175 70 1 1 70 179

It will be successful, printing "173 blocks compared" (It would print "Source and destination are not identical") if there were any differences.